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| author | Frank Baker <fbaker@hdfgroup.org> | 2000-03-09 22:37:20 (GMT) |
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| committer | Frank Baker <fbaker@hdfgroup.org> | 2000-03-09 22:37:20 (GMT) |
| commit | 65d0e1aa4d17679d0cd614eaaf553a3c34fd9522 (patch) | |
| tree | b44edc99bf0c33ea8c44684eeab12cf81998bece | |
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diff --git a/doc/html/Datatypes.html b/doc/html/Datatypes.html index 8d0b674..bc4a9e0 100644 --- a/doc/html/Datatypes.html +++ b/doc/html/Datatypes.html @@ -1,7 +1,7 @@ <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN"> <html> <head> - <title>The Data Type Interface (H5T)</title> + <title>Datatype Interface (H5T)</title> </head> <body bgcolor="#FFFFFF"> @@ -20,88 +20,72 @@ </td> <td valign=top align=right> And in this document, the - <a href="H5.user.html">HDF5 User's Guide</a>: - <a href="Files.html">Files</a> + <a href="H5.user.html"><strong>HDF5 User's Guide:</strong></a> <br> + <a href="Files.html">Files</a> <a href="Datasets.html">Datasets</a> - Data Types + Datatypes <a href="Dataspaces.html">Dataspaces</a> <a href="Groups.html">Groups</a> - <a href="References.html">References</a> <br> + <a href="References.html">References</a> <a href="Attributes.html">Attributes</a> <a href="Properties.html">Property Lists</a> <a href="Errors.html">Error Handling</a> + <br> <a href="Filters.html">Filters</a> + <a href="Palettes.html">Palettes</a> <a href="Caching.html">Caching</a> - <br> <a href="Chunking.html">Chunking</a> + <a href="MountingFiles.html">Mounting Files</a> + <br> + <a href="Performance.html">Performance</a> <a href="Debugging.html">Debugging</a> <a href="Environment.html">Environment</a> <a href="ddl.html">DDL</a> + <br> <a href="Ragged.html">Ragged Arrays</a> -<!-- -<hr> -And in this document, the -<a href="H5.user.html">HDF5 User's Guide</a>: - <a href="Attributes.html">H5A</a> - <a href="Datasets.html">H5D</a> - <a href="Errors.html">H5E</a> - <a href="Files.html">H5F</a> - <a href="Groups.html">H5G</a> - <a href="Properties.html">H5P</a> - <a href="References.html">H5R & H5I</a> - <a href="Ragged.html">H5RA</a> - <a href="Dataspaces.html">H5S</a> - <a href="Datatypes.html">H5T</a> - <a href="Filters.html">H5Z</a> - <a href="Caching.html">Caching</a> - <a href="Chunking.html">Chunking</a> - <a href="Debugging.html">Debugging</a> - <a href="Environment.html">Environment</a> - <a href="ddl.html">DDL</a> ---> </td></tr> </table> </center> <hr> - <h1>The Data Type Interface (H5T)</h1> + <h1>The Datatype Interface (H5T)</h1> <h2>1. Introduction</h2> - <p>The data type interface provides a mechanism to describe the + <p>The datatype interface provides a mechanism to describe the storage format of individual data points of a data set and is hopefully designed in such a way as to allow new features to be - easily added without disrupting applications that use the data - type interface. A dataset (the H5D interface) is composed of a + easily added without disrupting applications that use the + datatype interface. A dataset (the H5D interface) is composed of a collection or raw data points of homogeneous type organized according to the data space (the H5S interface). - <p>A data type is a collection of data type properties, all of + <p>A datatype is a collection of datatype properties, all of which can be stored on disk, and which when taken as a whole, provide complete information for data conversion to or from that - data type. The interface provides functions to set and query - properties of a data type. + datatype. The interface provides functions to set and query + properties of a datatype. - <p>A <em>data point</em> is an instance of a <em>data type</em>, + <p>A <em>data point</em> is an instance of a <em>datatype</em>, which is an instance of a <em>type class</em>. We have defined a set of type classes and properties which can be extended at a later time. The atomic type classes are those which describe - types which cannot be decomposed at the data type interface + types which cannot be decomposed at the datatype interface level; all other classes are compound. - <h2>2. General Data Type Operations</h2> + <h2>2. General Datatype Operations</h2> - <p>The functions defined in this section operate on data types as - a whole. New data types can be created from scratch or copied - from existing data types. When a data type is no longer needed + <p>The functions defined in this section operate on datatypes as + a whole. New datatypes can be created from scratch or copied + from existing datatypes. When a datatype is no longer needed its resources should be released by calling <code>H5Tclose()</code>. - <p>Data types come in two flavors: named data types and transient - data types. A named data type is stored in a file while the - transient flavor is independent of any file. Named data types + <p> Datatypes come in two flavors: named datatypes and transient + datatypes. A named datatype is stored in a file while the + transient flavor is independent of any file. Named datatypes are always read-only, but transient types come in three varieties: modifiable, read-only, and immutable. The difference between read-only and immutable types is that immutable types @@ -112,61 +96,61 @@ And in this document, the <dl> <dt><code>hid_t H5Tcreate (H5T_class_t <em>class</em>, size_t <em>size</em>)</code> - <dd>Data types can be created by calling this - function, where <em>class</em> is a data type class + <dd> Datatypes can be created by calling this + function, where <em>class</em> is a datatype class identifier. However, the only class currently allowed is - <code>H5T_COMPOUND</code> to create a new empty compound data - type where <em>size</em> is the total size in bytes of an - instance of this data type. Other data types are created with - <code>H5Tcopy()</code>. All functions that return data type + <code>H5T_COMPOUND</code> to create a new empty compound + datatype where <em>size</em> is the total size in bytes of an + instance of this datatype. Other datatypes are created with + <code>H5Tcopy()</code>. All functions that return datatype identifiers return a negative value for failure. <br><br> <dt><code>hid_t H5Topen (hid_t <em>location</em>, const char *<em>name</em>)</code> - <dd>A named data type can be opened by calling this function, - which returns a handle to the data type. The handle should - eventually be closed by calling <code>H5Tclose()</code> to - release resources. The named data type returned by this + <dd>A named datatype can be opened by calling this function, + which returns a datatype identifier. The identifier should + eventually be released by calling <code>H5Tclose()</code> to + release resources. The named datatype returned by this function is read-only or a negative value is returned for failure. The <em>location</em> is either a file or group - handle. + identifier. <br><br> <dt><code>herr_t H5Tcommit (hid_t <em>location</em>, const char *<em>name</em>, hid_t <em>type</em>)</code> - <dd>A transient data type (not immutable) can be committed to a - file and turned into a named data type by calling this + <dd>A transient datatype (not immutable) can be committed to a + file and turned into a named datatype by calling this function. The <em>location</em> is either a file or group - handle and when combined with <em>name</em> refers to a new - named data type. + identifier and when combined with <em>name</em> refers to a new + named datatype. <br><br> - <dt><code>hbool_t H5Tcommitted (hid_t <em>type</em>)</code> + <dt><code>htri_t H5Tcommitted (hid_t <em>type</em>)</code> <dd>A type can be queried to determine if it is a named type or a transient type. If this function returns a positive value then the type is named (that is, it has been committed perhaps by some other application). Datasets which return committed - data types with <code>H5Dget_type()</code> are able to share - the data type with other datasets in the same file. + datatypes with <code>H5Dget_type()</code> are able to share + the datatype with other datasets in the same file. <br><br> <dt><code>hid_t H5Tcopy (hid_t <em>type</em>)</code> - <dd>This function returns a modifiable transient data type + <dd>This function returns a modifiable transient datatype which is a copy of <em>type</em> or a negative value for - failure. If <em>type</em> is a dataset handle then the type - returned is a modifiable transient copy of the data type of + failure. If <em>type</em> is a dataset identifier then the type + returned is a modifiable transient copy of the datatype of the specified dataset. <br><br> <dt><code>herr_t H5Tclose (hid_t <em>type</em>)</code> - <dd>Releases resources associated with a data type. The data - type identifier should not be subsequently used since the + <dd>Releases resources associated with a datatype. The + datatype identifier should not be subsequently used since the results would be unpredictable. It is illegal to close an - immutable transient data type. + immutable transient datatype. <br><br> - <dt><code>hbool_t H5Tequal (hid_t <em>type1</em>, hid_t + <dt><code>htri_t H5Tequal (hid_t <em>type1</em>, hid_t <em>type2</em>)</code> <dd>Determines if two types are equal. If <em>type1</em> and <em>type2</em> are the same then this function returns @@ -175,12 +159,12 @@ And in this document, the <br><br> <dt><code>herr_t H5Tlock (hid_t <em>type</em>)</code> - <dd>A transient data type can be locked, making it immutable + <dd>A transient datatype can be locked, making it immutable (read-only and not closable). The library does this to all predefined types to prevent the application from inadvertently modifying or deleting (closing) them, but the application is - also allowed to do this for its own data types. Immutable - data types are closed when the library closes (either by + also allowed to do this for its own datatypes. Immutable + datatypes are closed when the library closes (either by <code>H5close()</code> or by normal program termination). </dl> @@ -190,8 +174,8 @@ And in this document, the smaller units at the API level. All atomic types have a common set of properties which are augmented by properties specific to a particular type class. Some of these properties also apply to - compound data types, but we discuss them only as they apply to - atomic data types here. The properties and the functions that + compound datatypes, but we discuss them only as they apply to + atomic datatypes here. The properties and the functions that query and set their values are: <dl> @@ -212,12 +196,12 @@ And in this document, the padding which may appear on either side of the actual value. If this property is reset to a smaller value which would cause the significant part of the data to extend beyond the edge of - the data type then the <code>offset</code> property is + the datatype then the <code>offset</code> property is decremented a bit at a time. If the offset reaches zero and the significant part of the data still extends beyond the edge - of the data type then the <code>precision</code> property is - decremented a bit at a time. Decreasing the size of a data - type may fail if the <code>H5T_FLOAT</code> bit fields would + of the datatype then the <code>precision</code> property is + decremented a bit at a time. Decreasing the size of a + datatype may fail if the <code>H5T_FLOAT</code> bit fields would extend beyond the significant part of the type. Adjusting the size of an <code>H5T_STRING</code> automatically adjusts the precision as well. On error, <code>H5Tget_size()</code> @@ -227,13 +211,13 @@ And in this document, the <dt><code>H5T_order_t H5Tget_order (hid_t <em>type</em>)</code> <dt><code>herr_t H5Tset_order (hid_t <em>type</em>, H5T_order_t <em>order</em>)</code> - <dd>All atomic data types have a byte order which describes how - the bytes of the data type are layed out in memory. If the + <dd>All atomic datatypes have a byte order which describes how + the bytes of the datatype are layed out in memory. If the lowest memory address contains the least significant byte of the datum then it is said to be <em>little-endian</em> or <code>H5T_ORDER_LE</code>. If the bytes are in the oposite order then they are said to be <em>big-endian</em> or - <code>H5T_ORDER_BE</code>. Some data types have the same byte + <code>H5T_ORDER_BE</code>. Some datatypes have the same byte order on all machines and are <code>H5T_ORDER_NONE</code> (like character strings). If <code>H5Tget_order()</code> fails then it returns <code>H5T_ORDER_ERROR</code> which is a @@ -244,14 +228,14 @@ And in this document, the <dt><code>size_t H5Tget_precision (hid_t <em>type</em>)</code> <dt><code>herr_t H5Tset_precision (hid_t <em>type</em>, size_t <em>precision</em>)</code> - <dd>Some data types occupy more bytes than what is needed to + <dd>Some datatypes occupy more bytes than what is needed to store the value. For instance, a <code>short</code> on a Cray is 32 significant bits in an eight-byte field. The <code>precision</code> property identifies the number of significant bits of a datatype and the <code>offset</code> property (defined below) identifies its location. The <code>size</code> property defined above represents the entire - size (in bytes) of the data type. If the precision is + size (in bytes) of the datatype. If the precision is decreased then padding bits are inserted on the MSB side of the significant bits (this will fail for <code>H5T_FLOAT</code> types if it results in the sign, @@ -321,8 +305,8 @@ And in this document, the <dd>Integer data can be signed two's complement (<code>H5T_SGN_2</code>) or unsigned (<code>H5T_SGN_NONE</code>). Whether data is signed or not - becomes important when converting between two integer data - types of differing sizes as it determines how values are + becomes important when converting between two integer + datatypes of differing sizes as it determines how values are truncated and sign extended. </dl> @@ -483,20 +467,20 @@ And in this document, the Otherwise new bits are filled according to the <code>msb</code> padding type. - <h3>3.6 Character and String Datatype Issues</h3> + <h3>3.6. Character and String Datatype Issues</h3> The <code>H5T_NATIVE_CHAR</code> and <code>H5T_NATIVE_UCHAR</code> - data types are actually numeric data (1-byte integers). If the + datatypes are actually numeric data (1-byte integers). If the application wishes to store character data, then an HDF5 - <em>string</em> data type should be derived from + <em>string</em> datatype should be derived from <code>H5T_C_S1</code> instead. <h4>Motivation</h4> - HDF5 defines at least three classes of data types: + HDF5 defines at least three classes of datatypes: integer data, floating point data, and character data. However, the C language defines only integer and - floating point data types; character data in C is + floating point datatypes; character data in C is overloaded on the 8- or 16-bit integer types and character strings are overloaded on arrays of those integer types which, by convention, are terminated with @@ -514,7 +498,7 @@ And in this document, the <h4>Usage</h4> To store <code>unsigned char s[256]</code> data as an - array of integer values, use the HDF5 data type + array of integer values, use the HDF5 datatype <code>H5T_NATIVE_UCHAR</code> and a data space that describes the 256-element array. Some other application that reads the data will then be able to read, say, a @@ -522,7 +506,7 @@ And in this document, the perform the numeric translation. To store <code>unsigned char s[256]</code> data as a - character string, derive a fixed length string data type + character string, derive a fixed length string datatype from <code>H5T_C_S1</code> by increasing its size to 256 characters. Some other application that reads the data will be able to read, say, a space padded string @@ -557,12 +541,12 @@ And in this document, the The C language uses the term <code>char</code> to represent one-byte numeric data and does not make - character strings a first-class data type. + character strings a first-class datatype. HDF5 makes a distinction between integer and character data and maps the C <code>signed char</code> (<code>H5T_NATIVE_CHAR</code>) and <code>unsigned char</code> (<code>H5T_NATIVE_UCHAR</code>) - data types to the HDF5 integer type class. + datatypes to the HDF5 integer type class. <h2>4. Properties of Opaque Types</h2> @@ -578,31 +562,31 @@ And in this document, the <h2>5. Properties of Compound Types</h2> - <p>A compound data type is similar to a <code>struct</code> in C + <p>A compound datatype is similar to a <code>struct</code> in C or a common block in Fortran: it is a collection of one or more atomic types or small arrays of such types. Each <em>member</em> of a compound type has a name which is unique within that type, and a byte offset that determines the first byte (smallest byte address) of that member in a compound datum. - A compound data type has the following properties: + A compound datatype has the following properties: <dl> <dt><code>H5T_class_t H5Tget_class (hid_t <em>type</em>)</code> - <dd>All compound data types belong to the type class + <dd>All compound datatypes belong to the type class <code>H5T_COMPOUND</code>. This property is read-only and is - defined when a data type is created or copied (see + defined when a datatype is created or copied (see <code>H5Tcreate()</code> or <code>H5Tcopy()</code>). <br><br> <dt><code>size_t H5Tget_size (hid_t <em>type</em>)</code> - <dd>Compound data types have a total size in bytes which is - returned by this function. All members of a compound data - type must exist within this size. A value of zero is returned + <dd>Compound datatypes have a total size in bytes which is + returned by this function. All members of a compound + datatype must exist within this size. A value of zero is returned for failure; all successful return values are positive. <br><br> <dt><code>int H5Tget_nmembers (hid_t <em>type</em>)</code> - <dd>A compound data type consists of zero or more members + <dd>A compound datatype consists of zero or more members (defined in any order) with unique names and which occupy non-overlapping regions within the datum. In the functions that follow, individual members are referenced by an index @@ -614,7 +598,7 @@ And in this document, the <dt><code>char *H5Tget_member_name (hid_t <em>type</em>, int <em>membno</em>)</code> <dd>Each member has a name which is unique among its siblings in - a compound data type. This function returns a pointer to a + a compound datatype. This function returns a pointer to a null-terminated copy of the name allocated with <code>malloc()</code> or the null pointer on failure. The caller is responsible for freeing the memory returned by this @@ -644,7 +628,7 @@ And in this document, the mapped to the linear address space of memory with respect to some reference order (the reference order is specified in natural language documentation which describes the compound - data type). The application which "invented" the type will + datatype). The application which "invented" the type will often use the identity permutation and other applications will use a permutation that causes the elements to be rearranged to the desired order. Only the first few elements of @@ -659,22 +643,22 @@ And in this document, the <br><br> <dt><code>hid_t H5Tget_member_type (hid_t <em>type</em>, int <em>membno</em>)</code> - <dd>Each member has its own data type, a copy of which is - returned by this function. The returned data type identifier + <dd>Each member has its own datatype, a copy of which is + returned by this function. The returned datatype identifier should be released by eventually calling <code>H5Tclose()</code> on that type. </dl> - <p>Properties of members of a compound data type are + <p>Properties of members of a compound datatype are defined when the member is added to the compound type (see <code>H5Tinsert()</code>) and cannot be subsequently modified. This makes it imposible to define recursive data structures. <a name="DTypes-PredefinedAtomic"> - <h2>6. Predefined Atomic Data Types</h2> + <h2>6. Predefined Atomic Datatypes</h2> </a> - <p>The library predefines a modest number of data types having + <p>The library predefines a modest number of datatypes having names like <code>H5T_<em>arch</em>_<em>base</em></code> where <em>arch</em> is an architecture name and <em>base</em> is a programming type name. New types can be derived from the @@ -723,7 +707,7 @@ And in this document, the <tr valign=top> <td><code>NATIVE</code></td> - <td>This architecture contains C-like data types for the + <td>This architecture contains C-like datatypes for the machine on which the library was compiled. The types were actually defined by running the <code>H5detect</code> program when the library was @@ -992,12 +976,14 @@ H5Tset_size (str80, 80); </table> </center> - <h2>7. Defining Compound Data Types</h2> - <p>Unlike atomic data types which are derived from other atomic - data types, compound data types are created from scratch. First, - one creates an empty compound data type and specifies it's total - size. Then members are added to the compound data type in any + + <h2>7. Defining Compound Datatypes</h2> + + <p>Unlike atomic datatypes which are derived from other atomic + datatypes, compound datatypes are created from scratch. First, + one creates an empty compound datatype and specifies it's total + size. Then members are added to the compound datatype in any order. <p>Usually a C struct will be defined to hold a data point in @@ -1016,11 +1002,11 @@ H5Tset_size (str80, 80); <p>Each member must have a descriptive name which is the key used to uniquely identify the member within the compound - data type. A member name in an HDF5 data type does not + datatype. A member name in an HDF5 datatype does not necessarily have to be the same as the name of the member in the C struct, although this is often the case. Nor does one need to - define all members of the C struct in the HDF5 compound data - type (or vice versa). + define all members of the C struct in the HDF5 compound + datatype (or vice versa). <p> <center> @@ -1028,7 +1014,7 @@ H5Tset_size (str80, 80); <caption align=bottom><h4>Example: A simple struct</h4></caption> <tr> <td> - <p>An HDF5 data type is created to describe complex + <p>An HDF5 datatype is created to describe complex numbers whose type is defined by the <code>complex_t</code> struct. @@ -1053,9 +1039,9 @@ H5Tinsert (complex_id, "imaginary", HOFFSET(complex_t,im), macro. However, data stored on disk does not require alignment, so unaligned versions of compound data structures can be created to improve space efficiency on disk. These unaligned compound - data types can be created by computing offsets by hand to + datatypes can be created by computing offsets by hand to eliminate inter-member padding, or the members can be packed by - calling <code>H5Tpack()</code> (which modifies a data type + calling <code>H5Tpack()</code> (which modifies a datatype directly, so it is usually preceded by a call to <code>H5Tcopy()</code>): @@ -1066,12 +1052,12 @@ H5Tinsert (complex_id, "imaginary", HOFFSET(complex_t,im), <tr> <td> <p>This example shows how to create a disk version of a - compound data type in order to store data on disk in - as compact a form as possible. Packed compound data - types should generally not be used to describe memory + compound datatype in order to store data on disk in + as compact a form as possible. Packed compound + datatypes should generally not be used to describe memory as they may violate alignment constraints for the architecture being used. Note also that using a - packed data type for disk storage may involve a higher + packed datatype for disk storage may involve a higher data conversion cost. <p><code><pre> hid_t complex_disk_id = H5Tcopy (complex_id); @@ -1089,9 +1075,9 @@ H5Tpack (complex_disk_id); <caption align=bottom><h4>Example: A flattened struct</h4></caption> <tr> <td> - <p>Compound data types that have a compound data type + <p>Compound datatypes that have a compound datatype member can be handled two ways. This example shows - that the compound data type can be flattened, + that the compound datatype can be flattened, resulting in a compound type with only atomic members. @@ -1125,13 +1111,13 @@ H5Tinsert (surf_id, "y-im", HOFFSET(surf_t,y.im), <p>However, when the <code>complex_t</code> is used often it becomes inconvenient to list its members over and over again. So the alternative approach to - flattening is to define a compound data type and then + flattening is to define a compound datatype and then use it as the type of the compound members, as is done here (the typedefs are defined in the previous examples). <p><code><pre> -hid_t complex_id, surf_id; /*hdf5 data types*/ +hid_t complex_id, surf_id; /*hdf5 datatypes*/ complex_id = H5Tcreate (H5T_COMPOUND, sizeof c); H5Tinsert (complex_id, "re", HOFFSET(complex_t,re), @@ -1148,33 +1134,1314 @@ H5Tinsert (surf_id, "y", HOFFSET(surf_t,y), complex_id); </table> </center> + + <a name="Datatypes_Enum"> </a> - <h2>8. <a href="DatatypesEnum.html">Enumeration Data Types</a></h2> + <h2>8. Enumeration Datatypes</h2> - An HDF5 enumeration data type is a 1:1 mapping between a set of + <h3>8.1. Introduction</h3> + + <p>An HDF enumeration datatype is a 1:1 mapping between a set of symbols and a set of integer values, and an order is imposed on the symbols by their integer values. The symbols are passed between the application and library as character strings and all the values for a particular enumeration type are of the same integer type, which is not necessarily a native type. + + <h3>8.2. Creation</h3> + + <p>Creation of an enumeration datatype resembles creation of a + compound datatype: first an empty enumeration type is created, + then members are added to the type, then the type is optionally + locked. + + <dl> + <dt><code>hid_t H5Tcreate(H5T_class_t <em>type_class</em>, + size_t <em>size</em>)</code> + <dd>This function creates a new empty enumeration datatype based + on a native signed integer type. The first argument is the + constant <code>H5T_ENUM</code> and the second argument is the + size in bytes of the native integer on which the enumeration + type is based. If the architecture does not support a native + signed integer of the specified size then an error is + returned. + + <pre> +/* Based on a native signed short */ +hid_t hdf_en_colors = H5Tcreate(H5T_ENUM, sizeof(short));</pre> + + + <dt><code>hid_t H5Tenum_create(hid_t <em>base</em>)</code> + <dd>This function creates a new empty enumeration datatype based + on some integer datatype <em>base</em> and is a + generalization of the <code>H5Tcreate()</code> function. This + function is useful when creating an enumeration type based on + some non-native integer datatype, but it can be used for + native types as well. + + <pre> +/* Based on a native unsigned short */ +hid_t hdf_en_colors_1 = H5Tenum_create(H5T_NATIVE_USHORT); + +/* Based on a MIPS 16-bit unsigned integer */ +hid_t hdf_en_colors_2 = H5Tenum_create(H5T_MIPS_UINT16); + +/* Based on a big-endian 16-bit unsigned integer */ +hid_t hdf_en_colors_3 = H5Tenum_create(H5T_STD_U16BE);</pre> + + + <dt><code>herr_t H5Tenum_insert(hid_t <em>etype</em>, const char + *<em>symbol</em>, void *<em>value</em>)</code> + <dd>Members are inserted into the enumeration datatype + <em>etype</em> with this function. Each member has a symbolic + name <em>symbol</em> and some integer representation + <em>value</em>. The <em>value</em> argument must point to a value + of the same datatype as specified when the enumeration type + was created. The order of member insertion is not important + but all symbol names and values must be unique within a + particular enumeration type. + + <pre> +short val; +H5Tenum_insert(hdf_en_colors, "RED", (val=0,&val)); +H5Tenum_insert(hdf_en_colors, "GREEN", (val=1,&val)); +H5Tenum_insert(hdf_en_colors, "BLUE", (val=2,&val)); +H5Tenum_insert(hdf_en_colors, "WHITE", (val=3,&val)); +H5Tenum_insert(hdf_en_colors, "BLACK", (val=4,&val));</pre> + + + <dt><code>herr_t H5Tlock(hid_t <em>etype</em>)</code> + <dd>This function locks a datatype so it cannot be modified or + freed unless the entire HDF5 library is closed. Its use is + completely optional but using it on an application datatype + makes that datatype act like a predefined datatype. + + <pre> +H5Tlock(hdf_en_colors);</pre> + + </dl> + + <h3>8.3. Integer Operations</h3> + + <p>Because an enumeration datatype is derived from an integer + datatype, any operation which can be performed on integer + datatypes can also be performed on enumeration datatypes. This + includes: + + <p> + <center> + <table> + <tr> + <td><code>H5Topen()</code></td> + <td><code>H5Tcreate()</code></td> + <td><code>H5Tcopy()</code></td> + <td><code>H5Tclose()</code></td> + </tr><tr> + <td><code>H5Tequal()</code></td> + <td><code>H5Tlock()</code></td> + <td><code>H5Tcommit()</code></td> + <td><code>H5Tcommitted()</code></td> + </tr><tr> + <td><code>H5Tget_class()</code></td> + <td><code>H5Tget_size()</code></td> + <td><code>H5Tget_order()</code></td> + <td><code>H5Tget_pad()</code></td> + </tr><tr> + <td><code>H5Tget_precision()</code></td> + <td><code>H5Tget_offset()</code></td> + <td><code>H5Tget_sign()</code></td> + <td><code>H5Tset_size()</code></td> + </tr><tr> + <td><code>H5Tset_order()</code></td> + <td><code>H5Tset_precision()</code></td> + <td><code>H5Tset_offset()</code></td> + <td><code>H5Tset_pad()</code></td> + </tr><tr> + <td><code>H5Tset_sign()</code></td> + </tr> + </table> + </center> + + <p>In addition, the new function <code>H5Tget_super()</code> will + be defined for all datatypes that are derived from existing + types (currently just enumeration types). + + <dl> + <dt><code>hid_t H5Tget_super(hid_t <em>type</em>)</code> + <dd>Return the datatype from which <em>type</em> is + derived. When <em>type</em> is an enumeration datatype then + the returned value will be an integer datatype but not + necessarily a native type. One use of this function would be + to create a new enumeration type based on the same underlying + integer type and values but with possibly different symbols. + + <pre> +hid_t itype = H5Tget_super(hdf_en_colors); +hid_t hdf_fr_colors = H5Tenum_create(itype); +H5Tclose(itype); + +short val; +H5Tenum_insert(hdf_fr_colors, "ouge", (val=0,&val)); +H5Tenum_insert(hdf_fr_colors, "vert", (val=1,&val)); +H5Tenum_insert(hdf_fr_colors, "bleu", (val=2,&val)); +H5Tenum_insert(hdf_fr_colors, "blanc", (val=3,&val)); +H5Tenum_insert(hdf_fr_colors, "noir", (val=4,&val)); +H5Tlock(hdf_fr_colors);</pre> + </dl> + + <h3>8.4. Type Functions</h3> + + <p>A small set of functions is available for querying properties + of an enumeration type. These functions are likely to be used + by browsers to display datatype information. + + <dl> + <dt><code>int H5Tget_nmembers(hid_t <em>etype</em>)</code> + <dd>When given an enumeration datatype <em>etype</em> this + function returns the number of members defined for that + type. This function is already implemented for compound + datatypes. + + <br><br> + <dt><code>char *H5Tget_member_name(hid_t <em>etype</em>, int + <em>membno</em>)</code> + <dd>Given an enumeration datatype <em>etype</em> this function + returns the symbol name for the member indexed by + <em>membno</em>. Members are numbered from zero to + <em>N</em>-1 where <em>N</em> is the return value from + <code>H5Tget_nmembers()</code>. The members are stored in no + particular order. This function is already implemented for + compound datatypes. If an error occurs then the null pointer + is returned. The return value should be freed by calling + <code>free()</code>. + + <br><br> + <dt><code>herr_t H5Tget_member_value(hid_t <em>etype</em>, int + <em>membno</em>, void *<em>value</em>/*out*/)</code> + <dd>Given an enumeration datatype <em>etype</em> this function + returns the value associated with the member indexed by + <em>membno</em> (as described for + <code>H5Tget_member_name()</code>). The value returned + is in the domain of the underlying integer + datatype which is often a native integer type. The + application should ensure that the memory pointed to by + <em>value</em> is large enough to contain the result (the size + can be obtained by calling <code>H5Tget_size()</code> on + either the enumeration type or the underlying integer type + when the type is not known by the C compiler. + + <pre> +int i, n = H5Tget_nmembers(hdf_en_colors); +for (i=0; i<n; i++) { + char *symbol = H5Tget_member_name(hdf_en_colors, i); + short val; + H5Tget_member_value(hdf_en_colors, i, &val); + printf("#%d %20s = %d\n", i, symbol, val); + free(symbol); +}</pre> + + <p> + Output: + <pre> +#0 BLACK = 4 +#1 BLUE = 2 +#2 GREEN = 1 +#3 RED = 0 +#4 WHITE = 3</pre> + </dl> + + <h3>8.5. Data Functions</h3> + + <p>In addition to querying about the enumeration type properties, + an application may want to make queries about enumerated + data. These functions perform efficient mappings between symbol + names and values. + + <dl> + <dt><code>herr_t H5Tenum_valueof(hid_t <em>etype</em>, const char + *<em>symbol</em>, void *<em>value</em>/*out*/)</code> + <dd>Given an enumeration datatype <em>etype</em> this function + returns through <em>value</em> the bit pattern associated with + the symbol name <em>symbol</em>. The <em>value</em> argument + should point to memory which is large enough to hold the result, + which is returned as the underlying integer datatype specified + when the enumeration type was created, often a native integer + type. + + <br><br> + <dt><code>herr_t H5Tenum_nameof(hid_t <em>etype</em>, void + *<em>value</em>, char *<em>symbol</em>, size_t + <em>size</em>)</code> + <dd>This function translates a bit pattern pointed to by + <em>value</em> to a symbol name according to the mapping + defined in the enumeration datatype <em>etype</em> and stores + at most <em>size</em> characters of that name (counting the + null terminator) to the <em>symbol</em> buffer. If the name is + longer than the result buffer then the result is not null + terminated and the function returns failure. If <em>value</em> + points to a bit pattern which is not in the domain of the + enumeration type then the first byte of the <em>symbol</em> + buffer is set to zero and the function fails. + + <pre> +short data[1000] = {4, 2, 0, 0, 5, 1, ...}; +int i; +char symbol[32]; + +for (i=0; i<1000; i++) { + if (H5Tenum_nameof(hdf_en_colors, data+i, symbol, + sizeof symbol))<0) { + if (symbol[0]) { + strcpy(symbol+sizeof(symbol)-4, "..."); + } else { + strcpy(symbol, "UNKNOWN"); + } + } + printf("%d %s\n", data[i], symbol); +} +printf("}\n");</pre> + + <p> + Output: + <pre> +4 BLACK +2 BLUE +0 RED +0 RED +5 UNKNOWN +1 GREEN +...</pre> + </dl> + + <h3>8.6. Conversion</h3> + + <p>Enumerated data can be converted from one type to another + provided the destination enumeration type contains all the + symbols of the source enumeration type. The conversion operates + by matching up the symbol names of the source and destination + enumeration types to build a mapping from source value to + destination value. For instance, if we are translating from an + enumeration type that defines a sequence of integers as the + values for the colors to a type that defines a different bit for + each color then the mapping might look like this: + + <p><img src="EnumMap.gif" alt="Enumeration Mapping"> + + <p>That is, a source value of <code>2</code> which corresponds to + <code>BLUE</code> would be mapped to <code>0x0004</code>. The + following code snippet builds the second datatype, then + converts a raw data array from one datatype to another, and + then prints the result. + + <pre> +/* Create a new enumeration type */ +short val; +hid_t bits = H5Tcreate(H5T_ENUM, sizeof val); +H5Tenum_insert(bits, "RED", (val=0x0001,&val)); +H5Tenum_insert(bits, "GREEN", (val=0x0002,&val)); +H5Tenum_insert(bits, "BLUE", (val=0x0004,&val)); +H5Tenum_insert(bits, "WHITE", (val=0x0008,&val)); +H5Tenum_insert(bits, "BLACK", (val=0x0010,&val)); + +/* The data */ +short data[6] = {1, 4, 2, 0, 3, 5}; + +/* Convert the data from one type to another */ +H5Tconvert(hdf_en_colors, bits, 5, data, NULL, plist_id); + +/* Print the data */ +for (i=0; i<6; i++) { + printf("0x%04x\n", (unsigned)(data[i])); +}</pre> + + <p> + Output: + <pre> + +0x0002 +0x0010 +0x0004 +0x0001 +0x0008 +0xffff</pre> + + <p>If the source data stream contains values which are not in the + domain of the conversion map then an overflow exception is + raised within the library, causing the application defined + overflow handler to be invoked (see + <code>H5Tset_overflow()</code>). If no overflow handler is + defined then all bits of the destination value will be set. + + <p>The HDF library will not provide conversions between enumerated + data and integers although the application is free to do so + (this is a policy we apply to all classes of HDF datatypes). + However, since enumeration types are derived from + integer types it is permissible to treat enumerated data as + integers and perform integer conversions in that context. + + <h3>8.7. Symbol Order</h3> + + <p>Symbol order is determined by the integer values associated + with each symbol. When the integer datatype is a native type, + testing the relative order of two symbols is an easy process: + simply compare the values of the symbols. If only the symbol + names are available then the values must first be determined by + calling <code>H5Tenum_valueof()</code>. + + <pre> +short val1, val2; +H5Tenum_valueof(hdf_en_colors, "WHITE", &val1); +H5Tenum_valueof(hdf_en_colors, "BLACK", &val2); +if (val1 < val2) ...</pre> + + <p>When the underlying integer datatype is not a native type then + the easiest way to compare symbols is to first create a similar + enumeration type that contains all the same symbols but has a + native integer type (HDF type conversion features can be used to + convert the non-native values to native values). Once we have a + native type we can compare symbol order as just described. If + <code>foreign</code> is some non-native enumeration type then a + native type can be created as follows: + + <pre> +int n = H5Tget_nmembers(foreign); +hid_t itype = H5Tget_super(foreign); +void *val = malloc(n * MAX(H5Tget_size(itype), sizeof(int))); +char *name = malloc(n * sizeof(char*)); +int i; + +/* Get foreign type information */ +for (i=0; i<n; i++) { + name[i] = H5Tget_member_name(foreign, i); + H5Tget_member_value(foreign, i, + (char*)val+i*H5Tget_size(foreign)); +} + +/* Convert integer values to new type */ +H5Tconvert(itype, H5T_NATIVE_INT, n, val, NULL, plist_id); + +/* Build a native type */ +hid_t native = H5Tenum_create(H5T_NATIVE_INT); +for (i=0; i<n; i++) { + H5Tenum_insert(native, name[i], ((int*)val)[i]); + free(name[i]); +} +free(name); +free(val);</pre> + + <p>It is also possible to convert enumerated data to a new type + that has a different order defined for the symbols. For + instance, we can define a new type, <code>reverse</code> that + defines the same five colors but in the reverse order. + + <pre> +short val; +int i; +char sym[8]; +short data[5] = {0, 1, 2, 3, 4}; + +hid_t reverse = H5Tenum_create(H5T_NATIVE_SHORT); +H5Tenum_insert(reverse, "BLACK", (val=0,&val)); +H5Tenum_insert(reverse, "WHITE", (val=1,&val)); +H5Tenum_insert(reverse, "BLUE", (val=2,&val)); +H5Tenum_insert(reverse, "GREEN", (val=3,&val)); +H5Tenum_insert(reverse, "RED", (val=4,&val)); + +/* Print data */ +for (i=0; i<5; i++) { + H5Tenum_nameof(hdf_en_colors, data+i, sym, sizeof sym); + printf ("%d %s\n", data[i], sym); +} + +puts("Converting..."); +H5Tconvert(hdf_en_colors, reverse, 5, data, NULL, plist_id); + +/* Print data */ +for (i=0; i<5; i++) { + H5Tenum_nameof(reverse, data+i, sym, sizeof sym); + printf ("%d %s\n", data[i], sym); +}</pre> + <p> - Details of enumeration data types and the related functions - are discussed on a separate - <a href="DatatypesEnum.html">Enumeration Data Types</a> page. + Output: + <pre> +0 RED +1 GREEN +2 BLUE +3 WHITE +4 BLACK +Converting... +4 RED +3 GREEN +2 BLUE +1 WHITE +0 BLACK</pre> + + <h3>8.8. Equality</h3> + + <p>The order that members are inserted into an enumeration type is + unimportant; the important part is the associations between the + symbol names and the values. Thus, two enumeration datatypes + will be considered equal if and only if both types have the same + symbol/value associations and both have equal underlying integer + datatypes. Type equality is tested with the + <code>H5Tequal()</code> function. + + <h3>8.9. Interacting with C's <code>enum</code> Type</h3> + + <p>Although HDF enumeration datatypes are similar to C + <code>enum</code> datatypes, there are some important + differences: - <h2>9. Sharing Data Types among Datasets</h2> + <p> + <center> + <table border width="80%"> + <tr> + <th>Difference</th> + <th>Motivation/Implications</th> + </tr> - <p>If a file has lots of datasets which have a common data type + <tr> + <td valign=top>Symbols are unquoted in C but quoted in + HDF.</td> + <td valign=top>This allows the application to manipulate + symbol names in ways that are not possible with C.</td> + </tr> + + <tr> + <td valign=top>The C compiler automatically replaces all + symbols with their integer values but HDF requires + explicit calls to do the same.</td> + <td valign=top>C resolves symbols at compile time while + HDF resolves symbols at run time.</td> + </tr> + + <tr> + <td valign=top>The mapping from symbols to integers is + <em>N</em>:1 in C but 1:1 in HDF.</td> + <td valign=top>HDF can translate from value to name + uniquely and large <code>switch</code> statements are + not necessary to print values in human-readable + format.</td> + </tr> + + <tr> + <td valign=top>A symbol must appear in only one C + <code>enum</code> type but may appear in multiple HDF + enumeration types.</td> + <td valign=top>The translation from symbol to value in HDF + requires the datatype to be specified while in C the + datatype is not necessary because it can be inferred + from the symbol.</td> + </tr> + + <tr> + <td valign=top>The underlying integer value is always a + native integer in C but can be a foreign integer type in + HDF.</td> + <td valign=top>This allows HDF to describe data that might + reside on a foreign architecture, such as data stored in + a file.</td> + </tr> + + <tr> + <td valign=top>The sign and size of the underlying integer + datatype is chosen automatically by the C compiler but + must be fully specified with HDF.</td> + <td valign=top>Since HDF doesn't require finalization of a + datatype, complete specification of the type must be + supplied before the type is used. Requiring that + information at the time of type creation was a design + decision to simplify the library.</td> + </tr> + </table> + </center> + + <p>The examples below use the following C datatypes: + + <p> + <table width="90%" bgcolor="white"> + <tr> + <td> + <code><pre> +/* English color names */ +typedef enum { + RED, + GREEN, + BLUE, + WHITE, + BLACK +} c_en_colors; + +/* Spanish color names, reverse order */ +typedef enum { + NEGRO + BLANCO, + AZUL, + VERDE, + ROJO, +} c_sp_colors; + +/* No enum definition for French names */ + </pre></code> + </td> + </tr> + </table> + + <h4>Creating HDF Types from C Types</h4> + + <p>An HDF enumeration datatype can be created from a C + <code>enum</code> type simply by passing pointers to the C + <code>enum</code> values to <code>H5Tenum_insert()</code>. For + instance, to create HDF types for the <code>c_en_colors</code> + type shown above: + + <p> + <table width="90%" bgcolor="white"> + <tr> + <td> + <code><pre> + +c_en_colors val; +hid_t hdf_en_colors = H5Tcreate(H5T_ENUM, sizeof(c_en_colors)); +H5Tenum_insert(hdf_en_colors, "RED", (val=RED, &val)); +H5Tenum_insert(hdf_en_colors, "GREEN", (val=GREEN,&val)); +H5Tenum_insert(hdf_en_colors, "BLUE", (val=BLUE, &val)); +H5Tenum_insert(hdf_en_colors, "WHITE", (val=WHITE,&val)); +H5Tenum_insert(hdf_en_colors, "BLACK", (val=BLACK,&val));</pre></code> + </td> + </tr> + </table> + + <h4>Name Changes between Applications</h4> + + <p>Occassionally two applicatons wish to exchange data but they + use different names for the constants they exchange. For + instance, an English and a Spanish program may want to + communicate color names although they use different symbols in + the C <code>enum</code> definitions. The communication is still + possible although the applications must agree on common terms + for the colors. The following example shows the Spanish code to + read the values assuming that the applications have agreed that + the color information will be exchanged using Enlish color + names: + + <p> + <table width="90%" bgcolor="white"> + <tr> + <td> + <code><pre> + +c_sp_colors val, data[1000]; +hid_t hdf_sp_colors = H5Tcreate(H5T_ENUM, sizeof(c_sp_colors)); +H5Tenum_insert(hdf_sp_colors, "RED", (val=ROJO, &val)); +H5Tenum_insert(hdf_sp_colors, "GREEN", (val=VERDE, &val)); +H5Tenum_insert(hdf_sp_colors, "BLUE", (val=AZUL, &val)); +H5Tenum_insert(hdf_sp_colors, "WHITE", (val=BLANCO, &val)); +H5Tenum_insert(hdf_sp_colors, "BLACK", (val=NEGRO, &val)); + +H5Dread(dataset, hdf_sp_colors, H5S_ALL, H5S_ALL, H5P_DEFAULT, data);</pre></code> + </td> + </tr> + </table> + + + <h4>Symbol Ordering across Applications</h4> + + <p>Since symbol ordering is completely determined by the integer values + assigned to each symbol in the <code>enum</code> definition, + ordering of <code>enum</code> symbols cannot be preserved across + files like with HDF enumeration types. HDF can convert from one + application's integer values to the other's so a symbol in one + application's C <code>enum</code> gets mapped to the same symbol + in the other application's C <code>enum</code>, but the relative + order of the symbols is not preserved. + + <p>For example, an application may be defined to use the + definition of <code>c_en_colors</code> defined above where + <code>WHITE</code> is less than <code>BLACK</code>, but some + other application might define the colors in some other + order. If each application defines an HDF enumeration type based + on that application's C <code>enum</code> type then HDF will + modify the integer values as data is communicated from one + application to the other so that a <code>RED</code> value + in the first application is also a <code>RED</code> value in the + other application. + + <p>A case of this reordering of symbol names was also shown in the + previous code snippet (as well as a change of language), where + HDF changed the integer values so 0 (<code>RED</code>) in the + input file became 4 (<code>ROJO</code>) in the <code>data</code> + array. In the input file, <code>WHITE</code> was less than + <code>BLACK</code>; in the application the opposite is true. + + <p>In fact, the ability to change the order of symbols is often + convenient when the enumeration type is used only to group + related symbols that don't have any well defined order + relationship. + + <h4>Internationalization</h4> + + <p>The HDF enumeration type conversion features can also be used + to provide internationalization of debugging output. A program + written with the <code>c_en_colors</code> datatype could define + a separate HDF datatype for languages such as English, Spanish, + and French and cast the enumerated value to one of these HDF + types to print the result. + + <p> + <table width="90%" bgcolor="white"> + <tr> + <td> + <code><pre> + +c_en_colors val, *data=...; + +hid_t hdf_sp_colors = H5Tcreate(H5T_ENUM, sizeof val); +H5Tenum_insert(hdf_sp_colors, "ROJO", (val=RED, &val)); +H5Tenum_insert(hdf_sp_colors, "VERDE", (val=GREEN, &val)); +H5Tenum_insert(hdf_sp_colors, "AZUL", (val=BLUE, &val)); +H5Tenum_insert(hdf_sp_colors, "BLANCO", (val=WHITE, &val)); +H5Tenum_insert(hdf_sp_colors, "NEGRO", (val=BLACK, &val)); + +hid_t hdf_fr_colors = H5Tcreate(H5T_ENUM, sizeof val); +H5Tenum_insert(hdf_fr_colors, "OUGE", (val=RED, &val)); +H5Tenum_insert(hdf_fr_colors, "VERT", (val=GREEN, &val)); +H5Tenum_insert(hdf_fr_colors, "BLEU", (val=BLUE, &val)); +H5Tenum_insert(hdf_fr_colors, "BLANC", (val=WHITE, &val)); +H5Tenum_insert(hdf_fr_colors, "NOIR", (val=BLACK, &val)); + +void +nameof(lang_t language, c_en_colors val, char *name, size_t size) +{ + switch (language) { + case ENGLISH: + H5Tenum_nameof(hdf_en_colors, &val, name, size); + break; + case SPANISH: + H5Tenum_nameof(hdf_sp_colors, &val, name, size); + break; + case FRENCH: + H5Tenum_nameof(hdf_fr_colors, &val, name, size); + break; + } +}</pre></code> + </td> + </tr> + </table> + + <h3>8.10. Goals That Have Been Met</h3> + + <p>The main goal of enumeration types is to provide communication + of enumerated data using symbolic equivalence. That is, a + symbol written to a dataset by one application should be read as + the same symbol by some other application. + + <p> + <table width="90%"> + <tr> + <td valign=top><b>Architecture Independence</b></td> + <td valign=top>Two applications shall be able to exchange + enumerated data even when the underlying integer values + have different storage formats. HDF accomplishes this for + enumeration types by building them upon integer types.</td> + </tr> + + <tr> + <td valign=top><b>Preservation of Order Relationship</b></td> + <td valign=top>The relative order of symbols shall be + preserved between two applications that use equivalent + enumeration datatypes. Unlike numeric values that have + an implicit ordering, enumerated data has an explicit + order defined by the enumeration datatype and HDF + records this order in the file.</td> + </tr> + + <tr> + <td valign=top><b>Order Independence</b></td> + <td valign=top>An application shall be able to change the + relative ordering of the symbols in an enumeration + datatype. This is accomplished by defining a new type with + different integer values and converting data from one type + to the other.</td> + </tr> + + <tr> + <td valign=top><b>Subsets</b></td> + <td valign=top>An application shall be able to read + enumerated data from an archived dataset even after the + application has defined additional members for the + enumeration type. An application shall be able to write + to a dataset when the dataset contains a superset of the + members defined by the application. Similar rules apply + for in-core conversions between enumerated datatypes.</td> + </tr> + + <tr> + <td valign=top><b>Targetable</b></td> + <td valign=top>An application shall be able to target a + particular architecture or application when storing + enumerated data. This is accomplished by allowing + non-native underlying integer types and converting the + native data to non-native data.</td> + </tr> + + <tr> + <td valign=top><b>Efficient Data Transfer</b></td> + <td valign=top>An application that defines a file dataset + that corresponds to some native C enumerated data array + shall be able to read and write to that dataset directly + using only Posix read and write functions. HDF already + optimizes this case for integers, so the same optimization + will apply to enumerated data. + </tr> + + <tr> + <td valign=top><b>Efficient Storage</b></td> + <td valign=top>Enumerated data shall be stored in a manner + which is space efficient. HDF stores the enumerated data + as integers and allows the application to chose the size + and format of those integers.</td> + </tr> + </table> + + + + + + + +<h2>9. Variable-length Datatypes</h2> + +<h3>9.1. Overview And Justification</h3> + +Variable-length (VL) datatypes are sequences of an existing datatype +(atomic, VL, or compound) which are not fixed in length from one dataset location +to another. In essence, they are similar to C character strings -- a sequence of +a type which is pointed to by a particular type of <em>pointer</em> -- although +they are implemented more closely to FORTRAN strings by including an explicit +length in the pointer instead of using a particular value to terminate the +sequence. + +<p> +VL datatypes are useful to the scientific community in many different ways, +some of which are listed below: +<ul> + <li>Ragged arrays: Multi-dimensional ragged arrays can be implemented with + the last (fastest changing) dimension being ragged by using a + VL datatype as the type of the element stored. (Or as a field in a + compound datatype.) + <li>Fractal arrays: If a compound datatype has a VL field of another compound + type with VL fields (a <em>nested</em> VL datatype), this can be used to + implement ragged arrays of ragged arrays, to whatever nesting depth is + required for the user. + <li>Polygon lists: A common storage requirement is to efficiently store arrays + of polygons with different numbers of vertices. VL datatypes can be + used to efficiently and succinctly describe an array of polygons with + different numbers of vertices. + <li>Character strings: Perhaps the most common use of VL datatypes will be to + store C-like VL character strings in dataset elements or as attributes + of objects. + <li>Indices: An array of VL object references could be used as an index to + all the objects in a file which contain a particular sequence of + dataset values. Perhaps an array something like the following: + <pre> + Value1: Object1, Object3, Object9 + Value2: Object0, Object12, Object14, Object21, Object22 + Value3: Object2 + Value4: <none> + Value5: Object1, Object10, Object12 + . + . + </pre> + <li>Object Tracking: An array of VL dataset region references can be used as + a method of tracking objects or features appearing in a sequence of + datasets. Perhaps an array of them would look like: + <pre> + Feature1: Dataset1:Region, Dataset3:Region, Dataset9:Region + Feature2: Dataset0:Region, Dataset12:Region, Dataset14:Region, + Dataset21:Region, Dataset22:Region + Feature3: Dataset2:Region + Feature4: <none> + Feature5: Dataset1:Region, Dataset10:Region, Dataset12:Region + . + . + </pre> +</ul> + + +<h3>9.2. Variable-length Datatype Memory Management</h3> + +With each element possibly being of different sequence lengths for a +dataset with a VL datatype, the memory for the VL datatype must be dynamically +allocated. Currently there are two methods of managing the memory for +VL datatypes: the standard C malloc/free memory allocation routines or a method +of calling user-defined memory management routines to allocate or free memory. +Since the memory allocated when reading (or writing) may be complicated to +release, an HDF5 routine is provided to traverse a memory buffer and free the +VL datatype information without leaking memory. + + +<h4>Variable-length datatypes cannot be divided</h4> + +VL datatypes are designed so that they cannot be subdivided by the library +with selections, etc. This design was chosen due to the complexities in +specifying selections on each VL element of a dataset through a selection API +that is easy to understand. Also, the selection APIs work on dataspaces, not +on datatypes. At some point in time, we may want to create a way for +dataspaces to have VL components to them and we would need to allow selections +of those VL regions, but that is beyond the scope of this document. + + +<h4>What happens if the library runs out of memory while reading?</h4> + +It is possible for a call to <code>H5Dread</code> to fail while reading in +VL datatype information if the memory required exceeds that which is available. +In this case, the <code>H5Dread</code> call will fail gracefully and any +VL data which has been allocated prior to the memory shortage will be returned +to the system via the memory management routines detailed below. +It may be possible to design a <em>partial read</em> API function at a +later date, if demand for such a function warrants. + + +<h4>Strings as variable-length datatypes</h4> + +Since character strings are a special case of VL data that is implemented +in many different ways on different machines and in different programming +languages, they are handled somewhat differently from other VL datatypes in HDF5. + +<p> +HDF5 has native VL strings for each language API, which are stored the +same way on disk, but are exported through each language API in a natural way +for that language. When retrieving VL strings from a dataset, users may choose +to have them stored in memory as a native VL string or in HDF5's <code>hvl_t</code> +struct for VL datatypes. + +<p> +VL strings may be created in one of two ways: by creating a VL datatype with +a base type of <code>H5T_NATIVE_ASCII</code>, <code>H5T_NATIVE_UNICODE</code>, +etc., or by creating a string datatype and setting its length to +<code>H5T_STRING_VARIABLE</code>. The second method is used to access +native VL strings in memory. The library will convert between the two types, +but they are stored on disk using different datatypes and have different +memory representations. + +<p> +Multi-byte character representations, such as UNICODE or <em>wide</em> +characters in C/C++, will need the appropriate character and string datatypes +created so that they can be described properly through the datatype API. +Additional conversions between these types and the current ASCII characters +will also be required. + +<p> +Variable-width character strings (which might be compressed data or some +other encoding) are not currently handled by this design. We will evaluate +how to implement them based on user feedback. + + +<h3>9.3. Variable-length Datatype API</h3> + +<h4>Creation</h4> + +VL datatypes are created with the <code>H5Tvlen_create()</code> function +as follows: +<dl> + <dd><em>type_id</em> = <code>H5Tvlen_create</code>(<em>hid_t</em> <code>base_type_id</code>); +</dl> + +<p> +The base datatype will be the datatype that the sequence is composed of, +characters for character strings, vertex coordinates for polygon lists, etc. +The base datatype specified for the VL datatype can be of any HDF5 datatype, +including another VL datatype, a compound datatype, or an atomic datatype. + + +<h4>Query base datatype of VL datatype</h4> + +It may be necessary to know the base datatype of a VL datatype before +memory is allocated, etc. The base datatype is queried with the +<code>H5Tget_super()</code> function, described in the H5T documentation. + + +<h4>Query minimum memory required for VL information</h4> + +It order to predict the memory usage that <code>H5Dread</code> may need +to allocate to store VL data while reading the data, the +<code>H5Dget_vlen_size()</code> function is provided: +<dl> + <dd><em>herr_t</em> + <code>H5Dget_vlen_buf_size</code>(<em>hid_t</em> <code>dataset_id</code>, + <em>hid_t</em> <code>type_id</code>, + <em>hid_t</em> <code>space_id</code>, + <em>hsize_t</em> *<code>size</code>) +</dl> + (This function is not implemented in Release 1.2.) + +<p> +This routine checks the number of bytes required to store the VL data from +the dataset, using the <code>space_id</code> for the selection in the dataset +on disk and the <code>type_id</code> for the memory representation of the +VL data in memory. The *<code>size</code> value is modified according to +how many bytes are required to store the VL data in memory. + + +<h4>Specifying how to manage memory for the VL datatype</h4> + +The memory management method is determined by dataset transfer properties +passed into the <code>H5Dread</code> and <code>H5Dwrite</code> functions +with the dataset transfer property list. + +<p> +Default memory management is set by using <code>H5P_DEFAULT</code> +for the dataset transfer property list identifier. +If <code>H5P_DEFAULT</code> is used with <code>H5Dread</code>, +the system <code>malloc</code> and <code>free</code> calls +will be used for allocating and freeing memory. +In such a case, <code>H5P_DEFAULT</code> should also be passed +as the property list identifier to <code>H5Dvlen_reclaim</code>. + +<p> +The rest of this subsection is relevant only to those who choose +<i>not</i> to use default memory management. + +<p> +The user can choose whether to use the +system <code>malloc</code> and <code>free</code> calls or +user-defined, or custom, memory management functions. +If user-defined memory management functions are to be used, +the memory allocation and free routines must be defined via +<code>H5Pset_vlen_mem_manager()</code>, as follows: +<dl> + <dd><em>herr_t</em> + <code>H5Pset_vlen_mem_manager</code>(<em>hid_t</em> <code>plist_id</code>, + <em>H5MM_allocate_t</em> <code>alloc</code>, + <em>void</em> *<code>alloc_info</code>, + <em>H5MM_free_t</em> <code>free</code>, + <em>void</em> *<code>free_info</code>) +</dl> + + +<p> +The <code>alloc</code> and <code>free</code> parameters +identify the memory management routines to be used. +If the user has defined custom memory management routines, +<code>alloc</code> and/or <code>free</code> should be set to make +those routine calls (i.e., the name of the routine is used as +the value of the parameter); +if the user prefers to use the system's <code> malloc</code> +and/or <code>free</code>, the <code>alloc</code> and +<code>free</code> parameters, respectively, should be set to +<code> NULL</code> +<p> +The prototypes for the user-defined functions would appear as follows: +<dl> + <dd><code>typedef</code> <em>void</em> + *(*<code>H5MM_allocate_t</code>)(<em>size_t</em> <code>size</code>, + <em>void</em> *<code>info</code>) ; + <dd><code>typedef</code> <em>void</em> + (*<code>H5MM_free_t</code>)(<em>void</em> *<code>mem</code>, + <em>void</em> *<code>free_info</code>) ; +</dl> + +<p> +The <code>alloc_info</code> and <code>free_info</code> parameters can be +used to pass along any required information to the user's memory management +routines. + +<p> +In summary, if the user has defined custom memory management +routines, the name(s) of the routines are passed in the +<code>alloc</code> and <code>free</code> parameters and the +custom routines' parameters are passed in the +<code>alloc_info</code> and <code>free_info</code> parameters. +If the user wishes to use the system <code> malloc</code> and +<code>free</code> functions, the <code>alloc</code> and/or +<code>free</code> parameters are set to <code> NULL</code> +and the <code>alloc_info</code> and <code>free_info</code> +parameters are ignored. + +<h4>Recovering memory from VL buffers read in</h4> + +The complex memory buffers created for a VL datatype may be reclaimed with +the <code>H5Dvlen_reclaim()</code> function call, as follows: +<dl> + <dd><em>herr_t</em> + <code>H5Dvlen_reclaim</code>(<em>hid_t</em> <code>type_id</code>, + <em>hid_t</em> <code>space_id</code>, + <em>hid_t</em> <code>plist_id</code>, + <em>void</em> *<code>buf</code>); +</dl> + +<p> +The <code>type_id</code> must be the datatype stored in the buffer, +<code>space_id</code> describes the selection for the memory buffer +to free the VL datatypes within, +<code>plist_id</code> is the dataset transfer property list which +was used for the I/O transfer to create the buffer, and +<code>buf</code> is the pointer to the buffer to free the VL memory within. +The VL structures (<code>hvl_t</code>) in the user's buffer are +modified to zero out the VL information after it has been freed. + +<p> +If nested VL datatypes were used to create the buffer, +this routine frees them from the bottom up, +releasing all the memory without creating memory leaks. + + +<h3>9.4. Code Examples</h3> + +The following example creates the following one-dimensional array +of size 4 of variable-length datatype. +<pre> + 0 10 20 30 + 11 21 31 + 22 32 + 33 +</pre> +Each element of the VL datatype is of H5T_NATIVE_UINT type. +<p> +The array is stored in the dataset and then read back into memory. +Default memory management routines are used for writing the VL data. +Custom memory management routines are used for reading the VL data and +reclaiming memory space. + +<center> +<table border align=center width="100%"> + <caption align=bottom><h4>Example: Variable-length Datatypes</h4></caption> + <tr> + <td> + <pre> +#include <hdf5.h> + +#define FILE "tvltypes.h5" +#define MAX(X,Y) ((X)>(Y)?(X):(Y)) + +/* 1-D dataset with fixed dimensions */ +#define SPACE1_NAME "Space1" +#define SPACE1_RANK 1 +#define SPACE1_DIM1 4 + +void *vltypes_alloc_custom(size_t size, void *info); +void vltypes_free_custom(void *mem, void *info); + +/**************************************************************** +** +** vltypes_alloc_custom(): VL datatype custom memory +** allocation routine. This routine just uses malloc to +** allocate the memory and increments the amount of memory +** allocated. +** +****************************************************************/ +void *vltypes_alloc_custom(size_t size, void *info) +{ + void *ret_value=NULL; /* Pointer to return */ + int *mem_used=(int *)info; /* Get the pointer to the memory used */ + size_t extra; /* Extra space needed */ + + /* + * This weird contortion is required on the DEC Alpha to keep the + * alignment correct. + */ + extra=MAX(sizeof(void *),sizeof(int)); + + if((ret_value=malloc(extra+size))!=NULL) { + *(int *)ret_value=size; + *mem_used+=size; + } /* end if */ + ret_value=((unsigned char *)ret_value)+extra; + return(ret_value); +} + +/**************************************************************** +** +** vltypes_free_custom(): VL datatype custom memory +** allocation routine. This routine just uses free to +** release the memory and decrements the amount of memory +** allocated. +** +****************************************************************/ +void vltypes_free_custom(void *_mem, void *info) +{ + unsigned char *mem; + int *mem_used=(int *)info; /* Get the pointer to the memory used */ + size_t extra; /* Extra space needed */ + + /* + * This weird contortion is required on the DEC Alpha to keep the + * alignment correct. + */ + extra=MAX(sizeof(void *),sizeof(int)); + + if(_mem!=NULL) { + mem=((unsigned char *)_mem)-extra; + *mem_used-=*(int *)mem; + free(mem); + } /* end if */ +} + +int main(void) +{ + hvl_t wdata[SPACE1_DIM1]; /* Information to write */ + hvl_t rdata[SPACE1_DIM1]; /* Information read in */ + hid_t fid1; /* HDF5 File IDs */ + hid_t dataset; /* Dataset ID */ + hid_t sid1; /* Dataspace ID */ + hid_t tid1; /* Datatype ID */ + hid_t xfer_pid; /* Dataset transfer property list ID */ + hsize_t dims1[] = {SPACE1_DIM1}; + uint i,j; /* counting variables */ + int mem_used=0; /* Memory used during allocation */ + herr_t ret; /* Generic return value */ + + + /* + * Allocate and initialize VL data to write + */ + for(i=0; i<SPACE1_DIM1; i++) { + + wdata[i].p=malloc((i+1)*sizeof(unsigned int)); + wdata[i].len=i+1; + for(j=0; j<(i+1); j++) + ((unsigned int *)wdata[i].p)[j]=i*10+j; + } /* end for */ + + /* + * Create file. + */ + fid1 = H5Fcreate(FILE, H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT); + + /* + * Create dataspace for datasets. + */ + sid1 = H5Screate_simple(SPACE1_RANK, dims1, NULL); + + /* + * Create a datatype to refer to. + */ + tid1 = H5Tvlen_create (H5T_NATIVE_UINT); + + /* + * Create a dataset. + */ + dataset=H5Dcreate(fid1,"Dataset1",tid1,sid1,H5P_DEFAULT); + + /* + * Write dataset to disk. + */ + ret=H5Dwrite(dataset,tid1,H5S_ALL,H5S_ALL,H5P_DEFAULT,wdata); + + /* + * Change to the custom memory allocation routines for reading VL data + */ + xfer_pid=H5Pcreate(H5P_DATASET_XFER); + + ret=H5Pset_vlen_mem_manager(xfer_pid,vltypes_alloc_custom, + &mem_used,vltypes_free_custom,&mem_used); + + /* + * Read dataset from disk. vltypes_alloc_custom and + * will be used to manage memory. + */ + ret=H5Dread(dataset,tid1,H5S_ALL,H5S_ALL,xfer_pid,rdata); + + /* + * Display data read in + */ + for(i=0; i<SPACE1_DIM1; i++) { + printf("%d-th element length is %d \n", i, (unsigned) rdata[i].len); + for(j=0; j<rdata[i].len; j++) { + printf(" %d ",((unsigned int *)rdata[i].p)[j] ); + } + printf("\n"); + } /* end for */ + + /* + * Reclaim the read VL data. vltypes_free_custom will be used + * to reclaim the space. + */ + ret=H5Dvlen_reclaim(tid1,sid1,xfer_pid,rdata); + + + /* + * Reclaim the write VL data. C language free function will be used + * to reclaim space. + */ + ret=H5Dvlen_reclaim(tid1,sid1,H5P_DEFAULT,wdata); + + /* + * Close Dataset + */ + ret = H5Dclose(dataset); + + /* + * Close datatype + */ + ret = H5Tclose(tid1); + + /* + * Close disk dataspace + */ + ret = H5Sclose(sid1); + + /* + * Close dataset transfer property list + */ + ret = H5Pclose(xfer_pid); + + /* + * Close file + */ + ret = H5Fclose(fid1); + +} + </pre> + </td> + </tr> +</table> +</center> + +And the output from this sample code would be as follows: + +<center> +<table border align=center width="100%"> + <caption align=bottom><h4>Example: Variable-length Datatypes, Sample Output</h4></caption> + <tr> + <td> + <pre> +0-th element length is 1 +0 +1-th element length is 2 +10 11 +2-th element length is 3 +20 21 22 +3-th element length is 4 +30 31 32 33 + </pre> + </td> + </tr> +</table> +</center> + +<p> +For further samples of VL datatype code, see the tests in <code>test/tvltypes.c</code> +in the HDF5 distribution. + + + + + <h2>10. Sharing Datatypes among Datasets</h2> + + <p>If a file has lots of datasets which have a common datatype then the file could be made smaller by having all the datasets - share a single data type. Instead of storing a copy of the data - type in each dataset object header, a single data type is stored + share a single datatype. Instead of storing a copy of the + datatype in each dataset object header, a single datatype is stored and the object headers point to it. The space savings is - probably only significant for datasets with a compound data type - since the simple data types can be described with just a few + probably only significant for datasets with a compound datatype + since the simple datatypes can be described with just a few bytes anyway. - <p>To create a bunch of datasets that share a single data type - just create the datasets with a committed (named) data type. + <p>To create a bunch of datasets that share a single datatype + just create the datasets with a committed (named) datatype. <p> <center> @@ -1182,9 +2449,9 @@ H5Tinsert (surf_id, "y", HOFFSET(surf_t,y), complex_id); <caption align=bottom><h4>Example: Shared Types</h4></caption> <tr> <td> - <p>To create two datasets that share a common data type - one just commits the data type, giving it a name, and - then uses that data type to create the datasets. + <p>To create two datasets that share a common datatype + one just commits the datatype, giving it a name, and + then uses that datatype to create the datasets. <p><code><pre> hid_t t1 = ...some transient type...; @@ -1208,7 +2475,7 @@ hid_t dset4 = H5Dcreate (file, "dset4", t2, space, H5P_DEFAULT); </center> <a name="Datatypes-DataConversion"> - <h2>10. Data Conversion</h2> + <h2>11. Data Conversion</h2> </a> <p>The library is capable of converting data from one type to @@ -1219,8 +2486,8 @@ hid_t dset4 = H5Dcreate (file, "dset4", t2, space, H5P_DEFAULT); <p>In order to insure that data conversion exceeds disk I/O rates, common data conversion paths can be hand-tuned and optimized for performance. The library contains very efficient code for - conversions between most native data types and a few non-native - data types, but if a hand-tuned conversion function is not + conversions between most native datatypes and a few non-native + datatypes, but if a hand-tuned conversion function is not available, then the library falls back to a slower but more general conversion function. The application programmer can define additional conversion functions when the libraries @@ -1242,7 +2509,7 @@ hid_t dset4 = H5Dcreate (file, "dset4", t2, space, H5P_DEFAULT); to be considered for inclusion in future versions of the library. - <p>A conversion path contains a source and destination data type + <p>A conversion path contains a source and destination datatype and each path contains a <em>hard</em> conversion function and/or a <em>soft</em> conversion function. The only difference between hard and soft functions is the way in which the library @@ -1266,9 +2533,9 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, </pre></code> <p>The conversion function is called with the source and - destination data types (<em>src_type</em> and + destination datatypes (<em>src_type</em> and <em>dst_type</em>), path-constant data (<em>cdata</em>), the - number of instances of the data type to convert + number of instances of the datatype to convert (<em>nelmts</em>), a buffer which initially contains an array of data having the source type and on return will contain an array of data having the destination type (<em>buffer</em>), and a @@ -1354,8 +2621,8 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, of the destination for the data which is passed in through the <em>buffer</em> argument. It can be used to "fill in between the cracks". For instance, if the destination type is a - compound data type and we are initializing only part of the - compound data type from the source type then the background + compound datatype and we are initializing only part of the + compound datatype from the source type then the background buffer can be used to initialize the other part of the destination. </dl> @@ -1390,7 +2657,7 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, the library and used as the conversion function when no data transformation is necessary. The application can redefine this path by specifying a new hard conversion function with a - negative value for both the source and destination data types, + negative value for both the source and destination datatypes, but the library might not call the function under certain circumstances. @@ -1442,7 +2709,7 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, 13 unsigned char *dst = src; 14 cray_ushort2be_t *priv = NULL; 15 -16 switch (cdata->command) { +16 switch (cdata->command) { 17 case H5T_CONV_INIT: 18 /* 19 * We are being queried to see if we handle this @@ -1462,12 +2729,12 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, 33 * is larger than the source size, then we must 34 * process the elements from right to left. 35 */ -36 cdata->priv = priv = malloc (sizeof(cray_ushort2be_t)); -37 priv->dst_size = H5Tget_size (dst); -38 if (priv->dst_size>8) { -39 priv->direction = -1; +36 cdata->priv = priv = malloc (sizeof(cray_ushort2be_t)); +37 priv->dst_size = H5Tget_size (dst); +38 if (priv->dst_size>8) { +39 priv->direction = -1; 40 } else { -41 priv->direction = 1; +41 priv->direction = 1; 42 } 43 break; 44 @@ -1475,8 +2742,8 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, 46 /* 47 * Free private data. 48 */ -49 free (cdata->priv); -50 cdata->priv = NULL; +49 free (cdata->priv); +50 cdata->priv = NULL; 51 break; 52 53 case H5T_CONV_CONV: @@ -1484,8 +2751,8 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, 55 * Convert each element, watch out for overlap src 56 * with dst on the left-most element of the buffer. 57 */ -58 priv = (cray_ushort2be_t *)(cdata->priv); -59 if (priv->direction<0) { +58 priv = (cray_ushort2be_t *)(cdata->priv); +59 if (priv->direction<0) { 60 src += (nelmts - 1) * 8; 61 dst += (nelmts - 1) * dst_size; 62 } @@ -1518,7 +2785,7 @@ typedef herr_t (*H5T_conv_t)(hid_t <em>src_type</em>, </pre></code> <p>The <em>background</em> argument is ignored since - it's generally not applicable to atomic data types. + it's generally not applicable to atomic datatypes. </td> </tr> </table> @@ -1546,7 +2813,7 @@ H5Tregister(H5T_PERS_SOFT, "cus2be", <p>This causes it to be consulted for any conversion from an integer type to another integer type. The first argument is just a short identifier which will - be printed with the data type conversion statistics. + be printed with the datatype conversion statistics. </td> </tr> </table> @@ -1580,47 +2847,31 @@ H5Tregister(H5T_PERS_SOFT, "cus2be", </td> <td valign=top align=right> And in this document, the - <a href="H5.user.html">HDF5 User's Guide</a>: - <a href="Files.html">Files</a> + <a href="H5.user.html"><strong>HDF5 User's Guide:</strong></a> <br> + <a href="Files.html">Files</a> <a href="Datasets.html">Datasets</a> - Data Types + Datatypes <a href="Dataspaces.html">Dataspaces</a> <a href="Groups.html">Groups</a> - <a href="References.html">References</a> <br> + <a href="References.html">References</a> <a href="Attributes.html">Attributes</a> <a href="Properties.html">Property Lists</a> <a href="Errors.html">Error Handling</a> + <br> <a href="Filters.html">Filters</a> + <a href="Palettes.html">Palettes</a> <a href="Caching.html">Caching</a> - <br> <a href="Chunking.html">Chunking</a> + <a href="MountingFiles.html">Mounting Files</a> + <br> + <a href="Performance.html">Performance</a> <a href="Debugging.html">Debugging</a> <a href="Environment.html">Environment</a> <a href="ddl.html">DDL</a> + <br> <a href="Ragged.html">Ragged Arrays</a> -<!-- -<hr> -And in this document, the -<a href="H5.user.html">HDF5 User's Guide</a>: - <a href="Attributes.html">H5A</a> - <a href="Datasets.html">H5D</a> - <a href="Errors.html">H5E</a> - <a href="Files.html">H5F</a> - <a href="Groups.html">H5G</a> - <a href="Properties.html">H5P</a> - <a href="References.html">H5R & H5I</a> - <a href="Ragged.html">H5RA</a> - <a href="Dataspaces.html">H5S</a> - <a href="Datatypes.html">H5T</a> - <a href="Filters.html">H5Z</a> - <a href="Caching.html">Caching</a> - <a href="Chunking.html">Chunking</a> - <a href="Debugging.html">Debugging</a> - <a href="Environment.html">Environment</a> - <a href="ddl.html">DDL</a> ---> </td></tr> </table> </center> @@ -1630,9 +2881,10 @@ And in this document, the <address> <a href="mailto:hdfhelp@ncsa.uiuc.edu">HDF Help Desk</a> </address> + <!-- Created: Thu Dec 4 14:57:32 EST 1997 --> <!-- hhmts start --> -Last modified: Fri Jun 4 16:14:04 EDT 1999 +Last modified: 14 October 1999 <!-- hhmts end --> |